The invention relates to a fluid power cylinder comprising a housing, a piston in said cylinder adapted to run axially therein, a piston rod, said rod being connected with said piston and extending from the cylinder at one end thereof at least, said piston rod possessing on the external periphery thereof at least one surface section extending in the axial direction and having the form of a flat, a longitudinal groove being provided in said section, and a measuring scale received in said groove and being capable of being scanned by a sensor device integral with the housing.